No Right Angle Alignment in PCB Layout

Right-angle alignment is generally avoided in PCB wiring layout, and become one of the criteria to measure the quality of wiring layout. But do right angle-alignment influence the signal transmission?

In principle, right-angle alignment changes the width of the transmission line, resulting in discontinuities in impedance. In fact, not only the right-angle alignment, obtuse angle and acute angle alignment also can cause impedance changes.

The signal influence caused by right-angle alignment is mainly shown in three aspects: one is that the corners can be equivalent to capacitive loads on the transmission line which can slow the rising time; another is discontinuous impedance can cause signal reflections; the last is right-angle tip can generate electro magnetic interference.

The parasitic capacitance caused by the right angle of the transmission line can be calculated from the following formula: C=61W(Er)1/2/Z0. C refers to the corner’s equivalent capacitance (unit: pF), W refers to the trace width (unit: inch), εr refers to the dielectric constant of the medium, and Z0 is the characteristic impedance of the transmission line.

As the line width at right angles increases, the impedance there will be reduced, so there will definitely have a signal reflection phenomenon. We can calculate the equivalent impedance after the line width increases according to the impedance calculation formula of the transmission line. The reflection coefficient is then calculated according to the formula ρ=(Zs-Z0)/(Zs+Z0). Impedance changes due to the right-angle alignment are often between 7% -20%, so the maximum reflection coefficient is about 0.1. 

PCB Wave Soldering Process

Wave soldering is a batch soldering process used to make printed circuit boards. Wave soldering is used for through-hole printed circuit assembly and surface mounting.

In the process of wave soldering, the molten solder is transformed into the desired wave by mechanical bump or electromagnetic bump flowing that features jet flow. PCB board with assembled components must pass through the solder wave to allow mechanical and electrical soldering between component soldering points or PCB pad.

The main task is to pre-mold some components to make them meet the requirements of mounting, plugging and wave soldering.

The key steps in wave soldering process include component molding, component insertion or mounting, soldering and cooling by soldering waves. This means that molded components can be plugged on the PCB board as needed. Then PCB loaded with components is pushed into the wave soldering system by the transmission device. Next, solder flux is sprayed and PCB is preheated in the preheat zone. The final step comes with the wave soldering and cooling.

PCB Manufacturing Process

PCB manufacturing process contains a series of steps. This process requires advanced high-precision machines and is only cost-effective when daily PCB fabrication volume is large. 

The manufacturing process begins with specifying a PCB development software. The fully designed PCB is saved in Gerber format and converted into a film negative using a plotter. Place the film on a board made of plastic with copper layers on both sides. The film is placed on the copper layer and then radiated by UV light. The film layer is UV-sensitive in places where copper connection lines go through, but is UV-resistant in other parts, so the film in connection area can only adhere to the copper layer. 

The sequential process is an optical check of all plates for errors, and then layers are aligned on the same machine in the correct order. Fiberglass with glue is placed on each copper plate and plates are attached to each other in an aligned order. The entire board is heat-pressed by a mechanical control to bond the boards together with high precision.

After that is the process of drilling holes, copper is placed into the holes and the whole structure is cleaned up. The outer layers are prepared after finishing the inner layer process. To protect the copper in the outer layer from corrosion, a solder mask is applied and then cured under UV light, often, the surface color of PCB board is green. 

To make the process of components soldering easier, gold or silver is sometimes applied over the outer layer. Finally, the PCB board is cut, shaped, electrically tested, documented and packaged, thus the whole PCB manufacturing process is completed.

Tips for PCB DFM Problems

PCB design for manufacturing issues are very common during the whole board fabrication process. There are 5 tips for your reference in this article to avoid unnecessary issues in your next project.

1.Some pads on the PCB board are easy to peel off.

As shown in the picture below, the leftmost ground pad of this brush soldering pad is easy to peel off in the process of debugging or rear-end maintenance, which will make the entire board to be a scrapped PCB.

What cause this problem? We can easily see that the connection area between pad and ground is too large, so the heat conduction is very fast which cool the soldering process down very quickly, naturally it is easy to peel off when pulling.

Solution: a.Blind holes are drilled on similar pads to make them connect with adjacent layers and increase the pulling force on the board.b.Use teardrop alignment on similar brush soldering pads, the reason is the same as above.c.Fabricating narrow-middle window on such kind of pad, so that green oil on the pad will have a certain pull.

2.Ground pad shall be cross connected.

3.You'd better not drill holes on the button center. If you put on the button center, it won’t work for a long time because the center contact friction is relatively large.

4.Make sure the silkscreen on PCB board is clear and neat.

5.Generally, tin is used to painted on the metal positioning hole. To avoid assemble issue caused by tin, it is not recommended to expose copper on the back of metal positioning hole, if inevitable, just expose a little copper. 

How to Make a Printed Circuit Board

PCB fabrication capability becomes particularly important when dealing with high-frequency signals and weak signals which are two major difficulties in the field of microelectronics. So in this article, let’s discuss how to make a good PCB.

1.Clearly Design GoalsWhen receiving a new design task, clear the design goal is the first step. You need to make a standard board, a high-frequency board, small signal processing PCB, or a board with both high-frequency property and small signal processing?If it is an ordinary PCB, you only need to achieve reasonable layout and wiring, and ensure that mechanical dimensions can be exactly the same as per requirements. When there is a signal line over 40MHz on the board, you have to give careful consideration to these signal lines, such as crosstalk between lines and other issues.

2.Understand Requirements for Wiring LayoutDuring the wiring layout process, some special components will have special requirements, for example, analog signal amplifiers used for LOTI and APH require  a stable and small-ripple power.

3.Component Layout ConsiderationsThe first factor to be considered is the electrical performance, put components with close connection as nearer as possible. Especially for some high-speed lines, the layout should make it as short as possible, and the power signal and small signal devices should be separated.In the premise of achieving the circuit performance, you should consider the board’s mechanical size, the location of the socket, or if components’ placement is neat, beautiful, and easy to test, etc.

4.Wiring ConsiderationsWith the design conclusion of OTNI and star-shaped optical network, more boards with over 100MHz high-speed signal lines will need to be designed in the coming future. Any long signal path on a printed circuit board can be considered as a transmission line. If the transmission delay of the line is much shorter than the signal rise time, then the main reflection produced during the signal rise will be submerged. Overshooting, kickback and ringing no longer occur. For most current MOS circuits, the trace length can be measured in meters with no signal distortion since the rise time is much greater than the transmission delay. For integrated circuits, due to the faster edge speed, the length of the trace must be significantly shortened to preserve the signal integrity.

PCB Design Tips to Reduce Electromagnetic Interference

PCB design becomes more difficult since electronic equipment have been more sensible and required excellent capability to reduce noise and electromagnetic interference. Thus, how to improve the design becomes one of the key issues that many engineers are concerned about. This article describes some tips for reducing noise and electromagnetic interference when designing PCB boards.

1.High-speed chips are used in the key areas.

2.To reduce the upper and lower edge of the control circuit toggle rate, you can string a resistor.

3.Use the lowest frequency clock that meets system requirements.

4.The clock generator is as close as possible to the device using the clock. Quartz crystal oscillator housing shall be grounded.

5.Use the ground wire to enclose the clock area; the clock line should be short as far as possible.

6.I/O driver circuit should be as close to the board edge as possible. Signals should be filtered when entering the printed circuit board, so as signals from the high noise area. At the same time, it is necessary to use string termination resistor approach to reduce the signal reflection.

7.MCD unused end shall be connected to ground or defined as the output end.

8.Don’t keep the input end of gate circuit unconnected; positive input of unused op amp shall be grounded, while negative input should  be connected to the output end.

9.Printed circuit board should use the 45-degree fold line instead of the 90 one in order to reduce the launch and coupling of high-frequency signals.

10.Areas on printed circuit boards should be divided by the frequency and current characteristics of the switch; keep enough distance between noisy components and non-noisy components.

PCB Tinning

Printed circuit boards must be protected in several ways to ensure their effective performance. Since copper tends to be oxidized in the air, it is difficult to bond with solder and the conductive quality is lowered as the same time. Thus solder mask is widely used as a cheap way to cover untapped copper surfaces on PCB with the help of resins and paints.

PCB tinning is the process of applying a layer of tin on copper traces to prevent any possible degradation. Tin plating also helps to mask copper traces when the circuit board is etched. By doing this, the manufacturing process is simplified and unwanted etching is prevented thanks to the industrial process use highly concentrated solvents like sodium sulphate. 

PCB tinning is able to increase the current carrying capacity of traces for high-power rails. Choosing a thicker copper cladding for smaller high-current areas can make the circuit board more expensive. So PCB tinning is ideal for bridging high current paths. It is mainly done by concentrating the deposition with solder on a specific area.

PCB tinning process uses pure tin, or solder with a 60:40 mixture of lead and tin. 

There are many ways to tin a printed circuit board, but the most common one is to place the circuit board in a tin plating solution and introduce the tin electrode into a controlled current plating process. The second coating is done after the last fabrication step. 

The solder layer can be deposited either by hand or by a hot solder extruder that can deposit the solder blob on to the area. Tin itself will oxidize when exposed to the outside environment and therefore can not completely and reliably keep the printed circuit board unmasked, thereby applying a solder mask is necessary.